Mapping Vibronic Dynamics of Ultrafast Intersystem Crossing in an Earth-Abundant Ligand-Field Excited Complex

Intersystem crossing (ISC) is a central nonradiative pathway in transition-metal complexes, critically shaping their behavior in photocatalysis and optoelectronics. In 3d metal systems, ligand-field (LF, d-d) excited states are especially important, yet the mechanisms governing their ISC remain obscure because LF transitions are commonly masked by intense metal-to-ligand charge-transfer (MLCT) bands. This knowledge gap is significant, as LF states frequently participate in deactivation channels and directly influence the photochemical reactivity. Here, we focus on unraveling the ultrafast ISC dynamics in a carefully chosen model system with a simplified electronic structure: cobalt(III)-acetylacetonate, ([Co(acac)₃]), a d⁶ low-spin complex. Upon selective ¹A₁ → ¹T₁ ligand-field excitation promoting a t_2g → e_g * electron, heterodyne-detected transient grating measurements with ∼10 fs pulses reveal vibrational coherences decaying in ∼50 fs, matching ISC dynamics. Fourier analysis reveals both low- and high-frequency vibrational modes associated with Co-O stretching and Co-O-C bending that actively mediate the spin-state transition. Complementary two-dimensional electronic spectroscopy (2DES) disentangles overlapping signals and localizes vibrational activity near the ¹T₁ excited-state absorption. Density functional theory (DFT) and GPU-accelerated hierarchy equation of motion (HEOM) calculations confirm that vibronic coupling, in concert with spin-orbit coupling (SOC), enables rapid singlet-to-triplet conversion via dynamic modulation of excited-state energies and reorganization along key nuclear coordinates. These results reveal that following LF excitation vibronic coupling plays an important role in reshaping excited-state potential energy surfaces and facilitating ISC in systems where SOC alone is weak. This work establishes a mechanistic foundation for understanding and controlling excited-state pathways in LF-dominated 3d transition-metal complexes.